Blood Pump

This application is a continuation of U.S. application Ser. No. 17/637,264, filed on Feb. 22, 2022, now allowed, which application is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/EP2020/074371, Sep. 1, 2020, published as International Publication No. WO 2021/043776 A 1, which claims the benefit of the filing date of European Patent Application No. 19194971.8 filed Sep. 2, 2019, the disclosures of which are hereby incorporated by reference.

This invention relates to an intravascular blood pump, in particular an intravascular blood pump for percutaneous insertion into a patient's blood vessel, to support a blood flow in a patient's blood vessel. The blood pump has an improved drive unit.

Blood pumps of different types are known, such as axial blood pumps, centrifugal (i.e. radial) blood pumps or mixed-type blood pumps, where the blood flow is caused by both axial and radial forces. Intravascular blood pumps are inserted into a patient's vessel such as the aorta by means of a catheter. A blood pump typically comprises a pump casing having a blood flow inlet and a blood flow outlet connected by a passage. In order to cause a blood flow along the passage from the blood flow inlet to the blood flow outlet, an impeller or rotor is rotatably supported within the pump casing, with the impeller being provided with blades for conveying blood.

Blood pumps are typically driven by a drive unit, which can be an electric motor. For instance, WO 2017/162619 A 1 discloses an intravascular blood pump having an impeller which is magnetically coupled to an electric motor. The impeller comprises magnets which are disposed adjacent to electrically magnetized zones in the electric motor. Due to attracting forces between the magnets in the impeller and electrically magnetized zones in the motor, rotation of the motor is transmitted to the impeller. That is, the drive unit has a plurality of static posts arranged about the axis of rotation of the impeller, and each post carries a wire coil winding and acts as a magnetic core. A control unit sequentially supplies a voltage to the coil windings to create a rotating magnetic field by which the magnetically coupled impeller is rotated.

More specifically, the intravascular blood pump in WO 2017/162619 A1 comprises a pump casing with a blood flow inlet and a blood flow outlet, an impeller and a drive unit for rotating the impeller. By rotation of the impeller about an axis of rotation and inside of the pump casing, blood can be conveyed from the blood flow inlet to the blood flow outlet by blades of the impeller. The drive unit comprises six posts and a back plate connecting rear ends of the posts to act as a yoke. The posts are arranged in a circle around the axis of rotation, as seen in a plane which is perpendicular to the axis of rotation, wherein each of the posts has a longitudinal axis, which is parallel to said axis of rotation. The posts each have a shaft and an inclined head portion at the impeller-side end of the shaft pointing towards the impeller, the head portion extending radially beyond the shaft so as to form a shoulder which can act as an axial stop for a coil winding disposed around each of the posts. A control unit sequentially supplies a voltage to the coil windings to create a rotating magnetic field. The impeller comprises a magnetic structure which is arranged to interact with the rotating magnetic field such that the impeller follows its rotation.

In operation, neighboring posts may have different magnetization. As a result thereof, magnetic flux running through the posts tends to flow between those neighboring posts in avoidance of the impeller. Such magnetic flux is lost for the generation of torque. A disadvantage of the state of the art is that the head portions extending radially beyond the shafts have a particularly small distance to each other. Accordingly, there is a considerable parasitic magnetic flux between the head portions, which is lost for the generation of torque. While such parasitic flux can be countered by placing magnetically insulating material, such as magnets, between the head portions, the available space is extremely limited and the polarization of the magnets would have to change cyclically in order to achieve a reasonable insulation, which is difficult. It is an objective of the invention to improve the drive unit in this regard.

The blood pump of the present invention may correspond to the afore-mentioned blood pump. Accordingly, it may be an axial blood pump or a diagonal blood pump, which pumps partly axially and partly radially (the diameter of pure centrifugal blood pumps is usually too large for intravascular applications). However, according to one aspect of the invention, a front surface of the impeller-side end of at least one of the posts-preferably of each of the posts-comprises a concavity in which the front surface is inclined downwards towards a central area of the front surface so as to concentrate at least a part of the magnetic field lines running through the front surface.

Magnetic field lines of magnetic flux exiting from and entering into a surface of a component made of magnetic material run perpendicularly to the surface, i.e. they exit and enter the surface plane vertically. By providing the front surface of a post with a concavity, i.e. with a depression having areas in which the front surface is inclined downwards towards a center of the front surface, the magnetic field lines which are running into and out of the post through the front surface are forced to run closer towards the central axis of the post. As a result, since magnetic field lines never cross each other, they are concentrated in front of the impeller-side end of the post and directed towards the impeller as a bundle. Parasitic flux between neighboring posts is thereby reduced.

The inclination of the concavity is less than 90°, preferably between 0° and 30°, relative to the surface plane.

Preferably, the concavity extends up to a circumference of the front surface. In other words, the concavity may start out from an outer border of the front surface. This has the effect that also the outermost magnetic field lines are affected by the inclination of the concavity. The outermost magnetic field lines are the ones which have the greatest tendency of bridging over to a neighboring post. Therefore, the concavity is most effective if it extends up to the circumference of the front surface of the post.

It may be sufficient that the concavity extends up to the circumference of the front surface of the post on at least two, preferably exactly two, opposite sides of the front surface, namely on those sides which are closest to the neighboring posts. This may be advantageous particularly when the posts are e.g. cylindrically and, thus, circular in cross section. That is, the danger that magnetic field lines bridge over to neighboring posts is the greatest where the posts have little distance from each other. Therefore, the concavity is sufficiently effective if it extends up to the circumference of the front surface of the post only on the two sides which are positioned closest to the respective neighboring posts.

Nevertheless, it is preferred that the circumference of the concavity coincides with the circumference of the front surface. This way, due to the inclination of the concavity, the outermost magnetic field lines are directed towards the center of the front surface along the entire circumference of the front surface. As mentioned, the outermost magnetic field lines are the ones which have the greatest tendency of turning away in avoidance of the impeller. Therefore, the concavity is most effective if its circumference coincides with the circumference of the front surface.

The concavity may have a flat bottom, as it may be sufficient to direct the outermost magnetic field lines towards the center. Thus, at least a region at the circumference of the concavity is downwardly inclined. In this case, the concavity may have a straight-lined inclined side wall when viewed in a cross-sectional plane running vertically through the front surface or it may have a curved inclined side wall when viewed in a cross-sectional plane running vertically through the front surface. A curved inclined side wall having an inclination which increases towards the circumference of the concavity has the effect that the bundling effect on the outermost magnetic field lines is maximal.

Alternatively, the concavity may have a curved cross section with a curved bottom, rather than a flat bottom, when viewed in a cross-sectional plane running vertically through the front surface. This way, the centering effect on the magnetic field lines gradually decreases from the circumference of the concavity towards a center thereof.

Further alternatively, the concavity may have a triangular cross section when viewed in a cross-sectional plane running vertically through the front surface. This way, the maximum depth of the concavity may be increased. The deeper the concavity, the greater the distance is between the respective portion of the front surface of the post and the magnetic structure of the impeller, resulting in reduced axial magnetic forces being generated between the post and the impeller. In particular, the ratio between magnetic torque and axial magnetic force can be increased by reducing the axial magnetic force, said ratio being an important figure in the development of magnetically driven intravascular blood pumps. Said ratio is important because the magnetic flux that can be generated is generally limited, so that it is desirable to use as much as possible of it for torque generation. The technical effect of the concavity is a reduced axial force acting on the rotor in axial direction without losing motor power or, alternatively, an increase of motor power at the same total magnetic flux.

This ratio can be increased even further according to a preferred aspect of the invention by a downward inclination of the front surface within the concavity in a radially outer direction (in addition to being inclined downwardly towards a central area thereof). Thus, relative to the axis of rotation, a radial inner region of the front surface in the concavity protrudes axially beyond a radial outer region of the front surface in the concavity. A gain, the result thereof is that the maximum depth of the concavity is increased. As mentioned, the deeper the concavity is, the greater the distance is between the respective portion of the front surface of the post and the magnetic structure of the impeller, resulting in reduced axial magnetic forces being generated between the post and the impeller. Thus, the ratio between the magnetic torque and axial magnetic force can be further increased by the downward inclination of the front surface within the concavity in a radially outer direction.

Another important effect achieved by the downward inclination of the front surface in a radially outer direction is that the bundle of concentrated magnetic field lines is directed radially outward and, thus, impinges on the magnetic structure of the impeller also radially outward as compared to a horizontal front surface. This has a positive effect on the achievable magnetic torque. A gain, this results in an improved ratio between magnetic torque and axial magnetic force. Thus, the positive effect of the downward inclination of the front surface within the concavity in a radially outer direction on the ratio between the magnetic torque and axial magnetic force is twofold.

According to a preferred embodiment of the invention, the combination of an inclination of the front surface in the concavity in both directions, centrally and radially outward, leads to a concavity which is open towards a side surface of the post, namely towards a side surface that is located radially outward relative to the axis of rotation. Preferably, the posts have a triangular cross section with three side surfaces, wherein one of the three side surfaces is located radially outward relative to the axis of rotation in comparison to the other two side surfaces. In such a case, the concavity is open towards the one of the three side surface that is located on the radially outer side of the post.

In all of the aforementioned variations, the concavity may preferably have a maximum depth of between 0.05 mm and 0.3 mm.

According to another aspect of the invention, the post does not extend with its impeller-side end radially beyond the impeller-side end of the respective coil winding disposed around the post, wherein the term “radially” relates to a direction transverse, preferably perpendicular, to the longitudinal axis of the respective post. In other words, the posts do not have a particular head portion. Instead, the posts preferably have a constant cross section at least at their impeller-side end region, more preferably along their entire length.

An advantage of posts having no head portion is that magnetic losses due to parasitic flux between neighboring posts are reduced by a greater distance between the posts. The result is again that the ratio between the achievable magnetic torque and the magnetic axial forces between the drive unit and the impeller is increased as compared to the pump described in WO 2017/162619 A 1 where the posts extend with their impeller-side ends radially beyond the impeller-side end of the respective coil winding.

According to a further aspect of the invention, the posts may each comprise a soft magnetic material which is discontinuous in cross section transverse, preferably perpendicular, to a longitudinal axis of the respective post, said axis preferably being parallel to the axis of rotation, as is described in further detail in WO 2019/057636 A1. “Discontinuous” in the sense of the present invention means that the soft magnetic material as seen in any cross section transverse to the longitudinal axis is interrupted, separated, intersected or the like by means of insulating material or other materials or gaps in order to form strictly separated areas of soft magnetic material or areas that are interrupted but connected at a different location. In other words, the soft magnetic material of the posts is discontinuous in cross section transverse, preferably perpendicular, to a direction of magnetic flux caused by the respective coil winding in the post. Providing a discontinuous soft magnetic material in cross-sectional planes transverse to the direction of the magnetic flux reduces eddy currents. This further increases the effectiveness of the intravascular blood pump.

Preferably, at least one weld is provided at a surface () of the discontinuous soft magnetic material, the weld bridging at least one discontinuity regarding electrical conductivity in the discontinuous soft magnetic material. The weld enables the easy manufacture of a magnetic core or a part of it out of a discontinuous soft magnetic material. That is, when separating the magnetic core or the posts for the magnetic core out of a larger work piece of discontinuous soft magnetic material, the discontinuous soft magnetic material may delaminate or otherwise lose its integrity due to the machining forces which are applied to the work piece during the separating process. This is particularly critical due to the very little dimensions of the magnetic core and especially the posts thereof and may even occur when electrical discharge machining, especially electrical discharge machining by wire cutting, is used for separating the magnetic core, or the posts therefor, out of the work piece. By means of the welds, which are applied to the work piece prior to the separation step, the mechanical stability of the discontinuous material is improved. In the case that electrical discharge machining is used for cutting the magnetic core or posts out of the work piece, also the flow of electric current to the location of cutting is improved. The weld or welds may later form a part of the magnetic core or posts. In particular, an impeller-side end surface of the posts being oriented transverse to the axis of rotation exposes the discontinuous material. Accordingly, the weld or welds may be arranged on the impeller-side surface of the posts.

The drive unit may comprise a back plate connecting the rear ends of the posts. Like the posts, the back plate may comprise a discontinuous soft magnetic material. Since the magnetic flux in the back plate is substantially transverse or perpendicular to the axis of rotation, the soft magnetic material of the back plate may be made discontinuous in cross section parallel to the axis of rotation. Alternatively, the posts and the back plate may be made from a monoblock of discontinuous soft magnetic material such that the soft magnetic material of the back plate and the discontinuous soft magnetic material of the posts is discontinuous in the same direction, preferably discontinuous in cross section perpendicular to the axis of rotation. Apart from that, substantially all features and explanations mentioned above with respect to the discontinuous material of the posts are valid also for the back plate. However, the back plate may alternatively be formed of continuous, i.e. solid, soft magnetic material.

According to one preferred embodiment of a drive unit comprising a back plate which connects the rear ends of the posts, a material of at least one of the posts is integral with a material of an intermediate area of the back plate, wherein the intermediate area of the back plate is an area of the back plate situated between the posts. Preferably, all posts are connected integrally to the back plate in this way. In other words, at least one post and the back plate, preferably the entire magnetic core of the drive unit, can be made of a single block of material, which may also be referred to as a monoblock. An advantage of such a magnetic core is that magnetic resistance at the transition between the posts and the back plate is minimized and, thus, magnetic flux is improved. Further, a good mechanical rigidity of the transition between the posts and the back plate can be achieved.

According to another preferred embodiment of a drive unit comprising a back plate which connects the rear ends of the posts, at least one of the posts and preferably all of the posts contact the back plate with a rear end surface of the respective post. This provides the advantage that the quality of the magnetic connection between the posts and the back plate can be made independent of the quality of the mechanical fastening of the posts to the back plate. For instance, the posts may be mechanically fastened to the back plate in corresponding recesses in the back plate or by means of glue provided around the rear ends of the posts. Thus, a good magnetic connection and, thus, a good magnetic flux can be achieved directly via the rear end surfaces of the posts into the back plate without being forced to accept constraints regarding the mechanical properties of the mechanical connection between the posts and the back plate. Furthermore, a magnetic path for transmission of magnetic flux is established which may exist additionally to a circumferential transmission of magnetic flux in the case where the rear ends of the posts are received in appropriately sized recesses in the back plate.

Thus, in this case the posts may be magnetically connected to the back plate at a corresponding contact plane of the back plate. The contact plane is preferably arranged parallel to the rear end surfaces of the posts. Preferably, it is arranged perpendicular to the axis of rotation. Preferably, the full surface area of the rear end surfaces of the posts is in contact with the back plate. This significantly reduces the magnetic resistance of the connection between the posts and the back plate. An unevenness of the rear end surface and the contact plane of the back plate is preferably such that a resulting gap is not more than 10 μm.

The back plate, like the posts, is preferably made of a soft magnetic material, such as electrical steel (magnetic steel) or other material suitable for closing the magnetic flux circuit, preferably cobalt steel. The diameter of the back plate may be in the range of 3 mm to 9 mm, such as 5 mm or 6 mm to 7 mm. The thickness of the back plate may be in the range of 0.5 mm to 2.5 mm, such as 1.5 mm. The outer diameter of the blood pump may be in the range of 4 mm to 10 mm, preferably 7 mm. The outer diameter of the arrangement of the plurality of posts may be in the range of 3 mm to 8 mm, such as 4 mm to 7.5 mm, preferably 6.5 mm.

As stated above, the posts are made of a soft magnetic material such as electrical steel (magnetic steel). The posts and the back plate may be made of the same material. Preferably, the magnetic core of the drive unit, including the posts and the back plate, is made of cobalt steel. The use of the cobalt steel contributes to reducing the pump size, in particular the diameter. With the highest magnetic permeability and highest magnetic saturation flux density among all magnetic steels, cobalt steel produces the most magnetic flux for the same amount of material used.

The dimensions of the posts, in particular length and cross-sectional area, may vary and depend on various factors. In contrast to the dimensions of the blood pump, e.g. the outer diameter, which depend on the application of the blood pump, the dimensions of the posts are determined by electromagnetic properties, which are adjusted to achieve a desired performance of the drive unit. One of the factors is the flux density to be achieved through the smallest cross-sectional area of the posts. The smaller the cross-sectional area, the higher the necessary current is to achieve the desired magnetic flux. A higher current, however, generates more heat in the wire of the coil due to electrical resistance. Even more importantly, the stator material quickly saturates magnetically if the cross section of the posts is too small. That means, although “thin” posts are preferred to reduce the overall size, this would require high current and, thus, result in undesired heat. The heat generated in the wire also depends on the length and diameter of the wire used for the coil windings. A short wire length and a large wire diameter are preferred in order to minimize the winding loss (referred to as “copper loss” or “copper power loss” if copper wires are used, which is usually the case). In other words, if the wire diameter is small, more heat is generated compared to a thicker wire at the same current, a preferred wire diameter being e.g. 0.05 mm to 0.2 mm, such as 0.1 mm. Further factors influencing the post dimensions and the performance of the drive unit are the number of windings of the coil and the outer diameter of the windings, i.e. the post including the windings. A large number of windings may be arranged in more than one layer around each post, for instance, two or three layers may be provided. However, the higher the number of layers, the more heat will be generated due to the increased length of the wire in the outer layers having a larger winding diameter. The increased length of the wire may generate more heat due to the higher resistance of a long wire compared to a shorter one. Thus, a single layer of windings with a small winding diameter would be preferred, but due to the required power, more than one winding is usually provided.

A typical number of windings, which in turn depends on the length of the post, may be about 50 to about 150, e.g.or. Independent of the number of windings, the coil windings are made of an electrically conductive material, in particular metal, such as copper or silver. Silver may be preferred to copper because silver has an electrical resistance which is about 5% less than the electrical resistance of copper.

Preferably, at least one post, more preferably each post, has a triangular cross section transverse to a longitudinal axis of the post. Preferably, the cross section of the post is triangular over its entire length. Triangular posts can utilize the available space inside a pump housing to a high percentage as such posts can be densely packed around the axis of rotation. Preferably, one side of the triangle faces away from the axis of rotation and is curved. The curvature bends around the axis of rotation. The radius of the curvature preferably corresponds to a radius of an outer diameter defined by the plurality of posts arranged about the axis of rotation. By such curvature, a further augmentation of the use of the space inside a cylindrical pump housing can be achieved.

Referring to, a cross-sectional view of a blood pumpis illustrated. The blood pumpcomprises a pump casinghaving a blood flow inletand a blood flow outlet. The blood pumpis designed as an intravascular blood pump, also called a catheter pump, and is deployed into a patient's blood vessel by means of a catheter. The blood flow inletis at the end of a flexible cannulawhich may be placed through a heart valve, such as the aortic valve, during use. The blood flow outletis located in a side surface of the pump casingand may be placed in a heart vessel, such as the aorta. The blood pumpis electrically connected with an electric lineextending through the catheterfor supplying the blood pumpwith electric power in order to drive the pumpby means of a drive unit, as explained in more detail below.

If the blood pumpis intended to be used in long-term applications, i.e. in situations in which the blood pumpis implanted into the patient for several weeks or even months, electric power is preferably supplied by means of a battery. This allows a patient to be mobile because the patient is not connected to a base station by means of cables. The battery can be carried by the patient and may supply electric energy to the blood pump, e.g. wirelessly.

The blood is conveyed along a passageconnecting the blood flow inletand the blood flow outlet(blood flow indicated by arrows). An impelleris provided for conveying blood along the passageand is mounted to be rotatable about an axis of rotationwithin the pump casingby means of a first bearingand a second bearing. The axis of rotationis preferably the longitudinal axis of the impeller. Both bearings,are contact-type bearings in this embodiment. At least one of the bearings,could, however, be a non-contact-type bearing such as a magnetic or hydrodynamic bearing. The first bearingis a pivot bearing having spherical bearing surfaces that allow for rotational movement as well as pivoting movement to some degree. A pinis provided, forming one of the bearing surfaces. The second bearingis disposed in a supporting memberto stabilize the rotation of the impeller, the supporting memberhaving at least one openingfor the blood flow. Bladesare provided on the impellerfor conveying blood once the impellerrotates. Rotation of the impelleris caused by the drive unitwhich is magnetically coupled to a magnetat an end portion of the impeller. The illustrated blood pumpis a mixed-type blood pump, with the major direction of flow being axial. It will be appreciated that the blood pumpcould also be a purely axial blood pump, depending on the arrangement of the impeller, in particular the blades.

The blood pumpcomprises the impellerand the drive unit. The drive unitcomprises a plurality of posts, such as six posts, only two of which are visible in the cross-sectional view of. The postsare arranged parallel to the axis of rotation, more specifically, a longitudinal axis of each of the postsis parallel to the axis of rotation. One endof the postsis disposed adjacent to the impeller. Coil windingsare arranged about the posts. The coil windingsare sequentially controlled by a control to create a rotating magnetic field. A part of the control unit is the printed circuit boardwhich is connected to the electric line. The impeller has a magnet, which is formed as a multiple-piece magnet in this embodiment. The magnetis disposed at the end of the impellerfacing the drive unit. The magnetis arranged to interact with the rotating magnetic field so as to cause rotation of the impellerabout the axis of rotation.

In order to close the magnetic flux path, a back plateis located at the end of the postsopposite the impeller-side of the posts. The postsact as a magnetic core and are made of a suitable material, in particular a soft magnetic material, such as steel or a suitable alloy, in particular cobalt steel. Likewise, the back plateis made of a suitable soft magnetic material, such as cobalt steel. The back plateenhances the magnetic flux, which allows for reduction of the overall diameter of the blood pump, which is important for intravascular blood pumps. For the same purpose, a yoke, i.e. an additional impeller back plate, is provided in the impellerat a side of the magnetfacing away from the drive unit. The yokein this embodiment has a conical shape in order to guide the blood flow along the impeller. The yokemay be made of cobalt steel, too. One or more wash-out channels that extend towards the central bearingmay be formed in the yokeor the magnet.

shows a cross-sectional view of a first preferred embodiment of a drive unit-impeller arrangement for the blood pump according to. As can be seen in, the impeller-side endsof the postsdo not extend radially beyond the windings. Rather, the cross section of the postsis constant in the direction of a longitudinal axis LA of the posts. It is thus avoided that the postscome close to each other, as this could cause a partial magnetic short-circuit with the result of a reduced power of the electric motor of the blood pump.

The drive unit according tomay comprise at least two posts. The number of posts is preferably a multiple of three and, thus, may be three, nine or twelve. Alternatively, the number of postsmay be a multiple of two, such as two, four, six, eight, ten or twelve. Higher numbers of postsmay be possible. A number of six postsis preferred. Due to the cross-sectional view, only two postsare visible. The postsand the back plateform a magnetic coreof the drive unitwhich may have a diameter of less than 10 mm.

The postsmay, as shown, consist of a discontinuous soft magnetic material that is discontinuous in regard of electrical conductivity. The discontinuous soft magnetic material comprises a plurality of sheetswhich are made of a ferromagnetic material and which are laminated to each other. A direction of lamination is arranged in direction of the longitudinal axis LA of the postsand marked by an arrow DL. As shown, the postsare arranged in parallel to the axis of rotation.

A spaceris disposed around the posts. It is made of a magnetically inactive material and has the purpose of keeping the distance of the postsconstant at their impeller-side ends. The spacerwill be described in further detail in regard to. The impeller-side endsof the coil windingsextend up to the spacer.

At the other ends of the posts, the back plateis provided. According to the embodiment shown in, the back platehas recesses for receiving therein the posts. More specifically, it comprises a first layerwith openingsfor rear endsof the posts. The back platewill be described in further detail in regard to.

It is conceivable to realize embodiments of the blood pumpwith arbitrary combinations of the three above-mentioned features: no radial extension of the impeller-side endsof the posts beyond the impeller-side ends of the windings, provision of a magnetically inactive spacerbetween the posts, and back platewith recesses for receiving the rear endsof the posts.

show a side view and a perspective view of the magnetic coreof the drive unitof the drive unit-impeller arrangement according to. The postsand back plateof the magnetic coreare shown with a distance to the magnetic structureof the impeller. As can be seen, the front surfaceof each of the impeller-side endsof the postsis provided with a concavity. In this particular embodiment and in all embodiments described hereinafter, the concavity extends over the entire front surfacesuch that the circumference of the concavity coincides with the circumference of the front surface.

Thus, the inclination of the concavity extends up to the circumference of the front surface. The concavity has a triangular cross section when viewed in a cross-sectional plane running vertically through the front surface, said plane, in the embodiment shown, being perpendicular to the longitudinal axis of the respective post. This would be different in embodiments in which each of the front surfacesof the postsis inclined so as to form together a cone-shaped front side of the magnetic core, as described in WO2017/162619 A 1. That is, in WO2017/162619 A 1, the posts each have a shaft and an inclined head portion at the impeller-side end of the shaft. Also, the front surface of those head portions, albeit inclined, may be provided with the afore-described concavity in which the front surface is inclined downwards towards a central area of the front surface and has a triangular cross section when viewed in a cross-sectional plane running vertically through the front surface.

The inclination of the front surfacewithin the concavity downwards towards a central area of the front surface serves to concentrate and, thus, bundle the magnetic field lines running through the front surface, as will be explained further below in relation to. However, inside the concavity the front surfaceis not only inclined downward towards the central area of the front surfacebut is further inclined downwards in a radially outer direction relative to the axis of rotation. In other words, the radial inner region of the front surfacein the concavity protrudes axially beyond a radial outer region of the front surfacein the concavity. The purpose of the downward inclination radially outwards has the purpose of directing the magnetic field lines towards the outer periphery of the magnetic structureof the impeller, thereby increasing the lever arm by which the impelleris rotated and, thus, increasing the torque. Consequently, the inclination of the concavity both towards the center of the front surfaceas well as radially outwards results for a postwith a triangular cross section in that the concavity is open towards the side surface of the postwhich is located radially outwards relative to the axis of rotation. The point of maximum depth of the front surfaceis, thus, positioned at the outer circumference of the respective postand may range between 0.05 mm and 0.3 mm, preferably between 0.1 mm and 0.2 mm, and is most preferably about 0.2 mm.

shows schematically a winding off of six posts,of a magnetic core, such as the magnetic coreshown in. In order to create a rotating magnetic field, two aspects are important. First, some of the posts must be magnetized in a positive direction while the others are magnetized in a negative direction so that the magnetic fluid lines of the magnetic flux extend from the positive by magnetized posts through the magnetic structure of the impellerinto the negatively magnetized posts and further through the back plateback into the positively magnetized posts so as to create a closed magnetic field. Second, the direction of magnetization of the posts must be sequentially changed from post to post in a circumferential direction so as to drag the magnetic structureof the impellerrotationally about the axis of rotation. In order to achieve this, neighboring posts are magnetized in opposite directions by an appropriately directed current running through the coil windingsprovided around each of the posts. For instance, a first post may be magnetized positively, a second, neighboring post negatively, a third, neighboring post positively, a fourth, neighboring post again negatively, and so forth. In a preferred embodiment, however, there are always two neighboring posts magnetized in one direction to drag the magnetic structureof the impeller, and only one of the next following posts is magnetized oppositely. In the case of six posts, four postsare magnetized in one direction and two postsare magnetized in the opposite direction, as indicated inin which a winding off of the six posts is schematically displayed. As can further be seen from, the magnetic field linesextending through the concave front surfaceare concentrated due to the inclined surfaces in the concavity so that the magnetic field lines form a bundle. The danger of a short circuit in the sense that magnetic field linesbridge between neighboring postsandis, thus, minimized.

each show a side view on the impeller-side endof a post according to four different embodiments. The embodiment shown incorresponds to the previously described embodiment with a concavity having a circumference which coincides with the circumference of the front surface and having two inclined side wallswhich are inclined both downwards towards the central area of the front surfaceand radially outwards relative to the axis of rotation, as explained above. As a result, the concavity has a triangular cross section when viewed in any cross-sectional plane running vertically through the front surface, and is open on the radially outer side of the post.

The embodiment shown insubstantially corresponds to the embodiment ofexcept that it has a flat bottom. Thus, the triangular cross section of the concavity is limited to the radially inner side of the post relative to the axis of rotation. Further radially outwards, the cross section is trapezoidal. Thus, the bottomis flat and parallel to the general plane of the front surface, whereas the side wallsare straight-lined side walls with oppositely oriented inclinations.

In the embodiment shown in, the concavity has a curved inclined side wallso that the inclination of the concavity is maximal at the circumference of the front surface.